Voltage regulation circuits accept unregulated voltage and produce a constant regulated voltage output which rejects fluctuations of the input supply voltage and may also provide over voltage protection, over current protection, temperature regulation, and the like. One application for a voltage regulation circuit is for supplying power to digital magnetic tape write drivers. The write drivers output a substantially constant current which changes direction in response to a digital input signal. The current is converted by a write head into a magnetic field which imposes a field pattern on a passing magnetic tape. At times when no write signal is desired, the write drivers are switched off to prevent any current from reaching the write heads. The write drivers, therefore, appear as a switched load to the voltage regulation circuit.
In order to minimize power dissipation, the level of the regulated voltage supplied to the write drivers is reduced. However, the inductance and resistance of the power distribution system is relatively unchanged. Since the required write current is independent of regulated supply voltage, power distribution perturbances become an increasingly greater percentage of the write current. An additional problem in low dissipation voltage regulation circuits occurs as the ratio of the regulated output voltage to the unregulated input voltage is minimized. Typically, series monitoring elements such as current sense resistors are used to sense turn-on current and provide soft start during regulator turn-on. Minimizing the output-to-input voltage ratio precludes the use of series monitoring elements which may result in unacceptably high turn-on current. Such high turn-on currents may trip protection circuits in the unregulated input voltage or may even cause a failure of the unregulated input supply.
What is needed is a voltage regulation circuit that can operate at low voltages without permitting high turn-on current surges. The voltage regulation circuit should supply regulated voltage to a switched load such as magnetic tape write drivers.